The invention relates to a method, wherein an operating parameter of an internal combustion engine in a motor vehicle is adjusted, for example the rotation speed of the internal combustion engine. The internal combustion engine hereby drives an electric generator of the motor vehicle. The present invention also relates to a motor vehicle in which a generator is coupled via an energy management unit with an engine control unit. The engine control unit hereby regulates an operating parameter of an internal combustion engine in response to an electric load connected to the generator.
A method and a motor vehicle of the aforementioned type are known, for example from DE 10 2008 004 269 A1. The excitation current of a motor-driven generator in a motor vehicle, in particular a truck, is here adjusted in a first control loop, while an intermediate circuit voltage generated by the generator is monitored in a second control loop, and a desired value for the excitation control is then determined therefrom. Optionally, an idle speed of the motor driving the generator can also be increased to provide a required drive power for the generator.
DE 10 2008 002 152 A1 describes a method for operating a drive unit of a vehicle, wherein a pilot control value for a load torque is corrected. With the method, the pilot control value which is often known only to the consumer at the idle speed can be relied upon, which can then be corrected with a known operational relationship, so that the load torques are adapted to the higher rotation speeds. Using a correction value obviates the need to use memory-intensive engine characteristics for the rotation speed-dependent adaptation of a pilot control value.
DE 198 20 395 A1 describes a DF signal generated in the voltage regulator of an alternator of a motor vehicle that indicates the utilization of the generator. This relates to a generator clock ratio for setting the output voltage of the generator by adjusting the excitation current by way of a two-position control with a square-wave voltage having a variable pulse width. The generator voltage decreases with increasing supplied output power, which must be introduced into the generator shaft by way of a corresponding mechanical drive power. The output power of the generator, or more precisely its efficiency, is hereby load-dependent and temperature-dependent. Therefore, the absolute mechanical power currently required by the generator cannot be inferred directly from the DF signal of the voltage regulator. For example, when the DF signal indicates a utilization rate of 70%, this can mean that the generator also requires two different mechanical drive powers at two different temperatures of the generator.
A generator model can therefore be used for determining characteristic values of the generator for an accurate regulation of the drive power for the generator shaft of the electric generator generated by an internal combustion engine of a motor vehicle. Such generator model is known, for example, from DE 10 2005 012 052 A1. The mechanical drive power required from the generator for generating the electric power can thus be determined with such a generator model, for example, based on operating variables of the generator, such as the aforedescribed duty ratio in the excitation (DF signal), the exciting current and the generator voltage. Such a relatively complex computational model has the disadvantage that, due to inherent signal and calculating transit times when determining the current parameter values, the calculation of the power value for the drive power and the transmission of the power value to the engine control unit is always delayed in relation to an actual change of load on the generator. This may have the consequence that, for example when controlling idling of an internal combustion by an engine control unit based on such power value, the response is not fast enough when an electrical load at the generator suddenly increases. This causes a drop in the rotation speed, i.e. the rotation speed drops by an undesirably large amount. For example, when a vehicle occupant turns on the air conditioning while the motor vehicle is idling, a reduction in the engine RPM is audible. The motor controller increases the engine speed only when an increased power demand is also signaled by the generator model, and the engine control unit then increases a control setpoint for rotation speed control or power control. A particularly undesirable effect arises with a cyclically activated electrical consumer, such as a windshield wiper. Since the current value calculated with the generator model is also cyclically transmitted to the engine control unit, a dynamic, cyclical superposition of the idle controller with the generator load may result, which can lead to an oscillation in the rotation speed.